The recall of Tony Blair to the Chilcot inquiry, announced last week, is likely to prove the major credibility test for him and his record on Iraq – and for the inquiry itself.

The focus of the cross-examination, which will be held in public towards the end of next month, is his relationship with the attorney general, Lord Goldsmith, and his handling of Goldsmith's legal advice about going to war.

One eminent lawyer I have spoken to said: "He [Blair] just seemed to think he could treat Goldsmith as his private counsel, his personal brief, and not a senior law officer serving the government and parliament as a whole."

Why, for instance, did the attorney general never fully address the Cabinet on the legality of what British forces were ordered to do in Iraq? He attended the full meeting of Cabinet on March 13, 2003 - two days before the invasion - but was not asked to speak.

There is more than a whiff of a suspicion that the Chilcot panel has been prompted to recall Blair and other key witnesses, including his then foreign secretary Jack Straw, by less than flattering reports contained in the WikiLeaks revelations.

A US dispatch from a member of the Bush administration during a visit to UK, published by WikiLeaks at the end of November, said that he had received assurances from a British MoD official, John Day, the director of policy, that "US security interests would be protected" in the Chilcot report. This has led to concerns that the inquiry would be accused of being a whitewash - a charge which has stuck to both the Hutton and Butler reports on Iraq.

But whatever the reason, Chilcot has now decided to extend the deadline for publishing the report to the spring. And because of powerful representations made to him personally by members of the legal profession as well politicians, he will examine more closely the role and record of the attorney general, and ask why Blair and Straw apparently chose to ignore his legal advice when it didn't suit them.

Since the first round of interviews by Chilcot, several key documents have come to light. One is a memo from Lord Goldsmith in early January 2003 in which he states that a second UN resolution would be needed, in addition to UNSCR 1441 of the previous November, to make action against Iraq legal. Blair himself pencilled on the note, "I just don't understand this," to which an aide added, "Specifically said we did not need further advice on this matter".

By early March, Lord Goldsmith was writing that after consulting US colleagues he now realised that a second UN resolution would not be needed to support action after all - though the UK could still be open to international indictment.

Goldsmith put his new view in summary form, which was put in a folder for the Cabinet to peruse at its March 17 meeting. Goldsmith attended that meeting but was not invited to rehearse the legal argument, it is understood.

The inquiry is now being pressed to obtain all Lord Goldsmith's legal opinions on the war in full - including the long one where he thought it illegal without an enabling UN resolution, and the full argument about his change of mind two months later.

It is also being asked to inquire why the attorney general did not brief the Cabinet and parliament fully about the legal argument in the run-up to the incursion into Iraq.

This is particularly important because the legal officer at the Foreign Office, Elizabeth Wilmshurst, had written that the operation without UN mandate was illegal - as she eloquently explained at one of the Chilcot hearings - and resigned the moment battle was joined.